Solar cells can be broadly classified into types which include silicon solar cells, thin-film solar cells and compound solar cells. Among these, progress is being made in the commercial development of thin-film solar cells as an optical device applying thin-film technology due to the advantages that their fabrication process is simple and does not require much energy. Chalcopyrite-type thin-film solar cells are categorized as thin-film cells, and comprise a CIGS layer consisting of a chalcopyrite-type compound (Cu(In+Ga)Se2: CIGS) as a p-type light absorbing layer. By using a light absorbing layer formed with such a compound, and in particular when employed together with an alkali-metal-containing glass substrate, such as soda lime glass, it is known that the solar cell can attain a high photoelectric conversion efficiency. Furthermore, such a cell is not only highly reliable due to a significantly reduced photodegradation (progressing over time), which is caused by contamination or lattice defects, photosensitive over a broad range of absorption wavelengths including longer wavelength bands, and at a high level in absorption coefficient but also highly radiation resistant, whereby its research and development aimed at mass practical application is progressing.
The layer structure of a typical thin-film solar cell which comprises a CIGS layer as its light absorbing layer is illustrated in FIG. 1. Such a solar cell is constituted by, on a soda lime glass (SLG) substrate 1, a multi-layered structure 7, which comprises a backside electrode layer 2 consisting of a Mo metal layer which serves as the positive electrode, a dipped Na layer 3 for preventing Na unevenness originating from the SLG substrate 1, the above-mentioned CIGS light absorbing layer 4, an n-type buffer layer 5 and a transparent conductive electrode 6 serving as the negative electrode on the outermost layer.
When sunlight or some other light is incident on an upper light receiving portion of such a solar cell, pairs of an electron and a hole are generated near the p-n junction of the multi-layered structure 7, since it is excited by the irradiated light having a energy higher than the bandgap. The excited electron and hole arrive at the p-n junction by diffusion, whereby due to the internal electric field of the junction, electrons collect at the n region and holes collect at both the p regions, and are thereby separated. As a result, the n region has a negative charge and the p region a positive charge, thus causing an electric potential difference between the electrodes 8 and 9 provided in the respective regions. This electric potential difference acts as an electromotive force, whereby a photocurrent is achieved when a conductor is connected between the respective electrodes. This is the principle of solar cells.
FIG. 2 is a process diagram illustrating the fabrication steps of a chalcopyrite-type thin-film solar cell having the multi-layered structure illustrated in FIG. 1.
When fabricating the above-mentioned solar cell, a Mo electrode layer is deposited (Mo electrode layer deposition step: FIG. 2a) by sputtering a metal Mo target onto a clean glass substrate of SLG or the like.
Subsequently, each substrate formed with a Mo electrode layer is divided up into a desired size by laser cutting. (first scribing step: FIG. 2b).
Next, the substrate is cleaned by washing with water or the like to remove the shavings and other debris, followed by dipping the cleaned substrate in a dilute solution of an alkali metal compound, such as sodium sulfide (alkali metal compound adhesion step: FIG. 2c). A bilayer structure consisting of an In metal layer and a Cu—Ga alloy layer is then deposited by sputtering both an In metal target and a Cu—Ga alloy target (a deposition step of precursor for a light absorbing layer: FIG. 2d).
As illustrated in FIG. 2e, in a conventional method for obtaining a CIGS light absorbing layer, for example, whole substrates, each having an In layer and a Cu—Ga alloy layer which constitutes the precursor of the CIGS layer is contained in an annealing chamber, wherein the substrates are preheated at 100° C. for 10 minutes. After preheating, the temperature in the chamber is elevated to the temperature range of 500 to 520° C. while hydrogen selenide (H2Se) gas is charged into the chamber via a gas inlet tube inserted thereinto and circulates in the annealing chamber. By such an annealing treatment, the precursor consisting of a layered structure of an In layer and a Cu—Ga layer is transformed into a CIGS monolayer, during which time the dipped Na layer diffuses into the light absorbing layer and disappears. Once the thermal treatments are completed, the hydrogen selenide reaction gas is displaced by a purging gas, such as Ar, and the annealed workpiece is cooled (see Patent Document 1).
A CIGS layer deposited substrate which has been removed from the annealing chamber then undergoes a buffer layer deposition using an n-type semiconductor material, such as CdS, Zno, InS or the like, by chemical bath deposition as illustrated in FIG. 2f or sputtering.
The substrate on which a buffer layer has been deposited is then cut using laser irradiation or a metal needle (second scribing step: FIG. 2g).
Subsequently, by sputtering using a ZnO—Al alloy target, a transparent conductive layer consisting of a ZnOAl layer is deposited as the outermost layer (FIG. 2h).
Finally, the substrate on which a transparent conductive layer has been deposited is again cut using laser irradiation or a metal needle (third scribing step: FIG. 2i).
The thin-film solar cell consisting of the above layered structure can be obtained as single cells whose size is uniform as a result of the cutting processing, whereby a final product can be made into a flat integrated structure by connecting such single cells in series.
As mentioned above, it is known that when a CIGS light absorbing layer is used in combination with an alkali-metal-containing glass substrate (e.g. an SLG substrate), a high photoelectric conversion efficiency can be attained. This phenomenon was shown to be from sodium atoms in the SLG diffusing into the light absorbing layer, thereby promoting particle growth in the layer. That is, when CIGS is employed as the light absorbing layer, Cu(In+Ga)Se2 crystallization is promoted, whereby as a result of this an increase in the photoelectric conversion efficiency is achieved.
Examples of such a light absorbing layer formation technique which employs an alkali metal layer include the dry process described in Patent Documents 2 and 3. According to Patent Document 2, a predetermined amount of sodium selenide is precipitated using vapor deposition into the Mo metal layer of the backside electrode, and a CIGS semiconductor layer is formed on the top thereof by sputtering and annealing, whereby sodium is doped into the semiconductor layer. On the other hand, Patent Document 3 discloses that, when co-depositing a Cu(In+Ga)Se chalcopyrite structure semiconductor component, a compound consisting of a group Ia element and a group VIa element, such as Na2Se, Na2S or the like, is deposited simultaneously or in tandem, and then the annealing treatment is carried out.
However, in the light absorbing layer formation technique according to Patent Document 2, since the sodium selenide precipitated onto the Mo metal layer is hygroscopic, when exposed to the air after vapor-deposition the precipitated matter alters, which can cause peeling to occur between the backside electrode and the light absorbing layer. Furthermore, in the vapor deposition according to Patent Document 3, in addition to the problem of the alkali metal compound being hygroscopic, fresh problems arise, such as the increased size of the deposition apparatus and resultant cost increase for its equipment.
The above-mentioned problems are all peculiar to a dry process. To solve such problems, the present inventors proposed a wet process as described in Patent Document 4. FIG. 3 illustrates the outline of the light absorbing layer formation steps disclosed in Patent Document 4, wherein an alkali metal compound is formed by a wet process.
Explaining the light absorbing layer formation steps of Patent Document 4 by referring to FIG. 3, first, a Mo electrode layer is formed onto an SLG substrate by sputtering. Next, an alkali layer is formed onto the Mo electrode layer by dipping. The alkali layer is formed by dipping the substrate provided with a Mo electrode layer in an aqueous solution of 0.01 to 1% by weight of sodium sulfide dissolved in pure water, drying the dipped substrate by spin drying or the like, and then baking the dried substrate in air for 60 minutes to regulate the remaining moisture thereof. Subsequently, the alkali layer is subjected to sputter deposition using, in order, an In target followed by a Cu—Ga alloy target, whereby a layered precursor is formed consisting of an In metal layer and a Cu—Ga alloy layer. The substrate provided with a precursor layer is then subjected to a selenization treatment in a Se atmosphere having a predetermined temperature, whereby a CIGS light absorbing layer is formed. At this stage, the alkali layer disappears by diffusion into the light absorbing layer adjacent directly above.    Patent Document 1: Japanese Patent Laid-Open No. 2003-282908    Patent Document 2: Japanese Patent Laid-Open No. H08-222750    Patent Document 3: Japanese Patent Laid-Open No. H08-102546    Patent Document 4: WO 03/069684 (pamphlet)    Non-Patent Document 1: M. Bodegard et al., “The Influence of Sodium on the Grain Structure of CuInSe2 Films for Photovoltaic Applications”, Proc. 12th Eur. Photovoltaic Solar Conf. 1994
By employing the wet process of Patent Document 4 in place of the dry processes of Patent Documents 2 and 3, the problems peculiar to dry processes, such as the alkali metal compound being hygroscopic and an increased apparatus size, can be resolved. However, on the other hand, when an aqueous solution having a higher concentration of the alkali metal compound is used in order to improve photoelectric conversion efficiency, when the concentration exceeds 1.0% by weight, the adhesion between the Mo electrode layer and the CIGS light absorbing layer decreases, whereby degradation over time, such as layer peeling, is more likely to occur.
Furthermore, in a light absorbing layer which has an alkali layer as the next layer between itself and the Mo electrode layer, stain spots sometimes appear on the surface thereof after the precursor forming step and selenization step. For this reason there is the problem that the external appearance of a thin-film solar cell fabricated by undergoing a transparent electrode forming step is significantly blemished, whereby its commercial value decreases.
It is an object of the present invention to provide a method for fabricating a chalcopyrite-type thin-film solar cell, which has good adhesion between the electrode layer and the CIGS light absorbing layer, and which has a stable layered structure and does not have any problems with its external appearance even when the concentration of an alkali-metal solution for forming an alkali layer, whose purpose is to improve photoelectric conversion efficiency, is comparatively high.